1. Field
This invention generally relates to the casting of primarily aluminum-based light alloy foundry parts.
Various foundry techniques are known, essentially those applied from the top of the mold in gravity mode and from the bottom of the mold in low-pressure mode. Various types of molds, primarily sand and metal molds, are also known.
2. Description of the Related Art
The use of gravity casting in metal molds has advantages for the production of foundry parts such as aluminum-alloy cylinder blocks for motor vehicle combustion engines or the like. In particular, such a method is suitable for small and medium series, because it is highly modular and minimizes the use of chemically-bound sand by the use of metal die walls.
In comparison with the casting technique for green sand molds, the gravity casting technique for metal molds has the advantage of an investment cost that is progressive, adapted and adjustable according to the actual production requirements.
However, the method for gravity metal mold casting of cylinder blocks, as conventionally practiced, does not enable a sturdy product of high metallurgical quality to be obtained in areas of the part such as the crankshaft bearings (areas that are more sensitive in terms of fatigue strength) while maintaining adequate dimensional control of the internal shapes with respect to one another.
Indeed, if one of these objectives is achieved, it is always to the detriment of the other.
For example, in reference to FIG. 1 of the drawings, if the gravity casting of a cylinder block in a V-shape is performed, with the crankshaft bearings PV in the upper portion, the situation is especially favorable for dimensional control of the barrels, in particular when liners inserted during casting are to be overmolded.
Indeed, the base of the mold makes enables all of the devices for guiding the metal pins, which form the barrels, to be placed in direct contact with the solidified alloy, or the metal pins that serve as a support for the liners CH to be placed on these barrel pins and themselves overmolded by the liquid alloy.
Similarly, this mold base can serve as a very practical support for the positioning of internal cores such as those intended to enable water to circulate.
However, it should be noted that these advantages of upwardly casting the block with crankshaft bearings are limited by the fact that, since the crankshaft bearings are under the risers MA, their metallurgic quality (in particular in terms of microporosity), mechanical characteristics and fatigue strength are significantly reduced with respect to what could be obtained with a faster cooling of the alloy.
If, on the other hand, the cylinder block is cast with the mold positioned in the other direction (i.e. with the crankshaft bearings downward) in order to promote the production of microconstructions and improved properties in the critical areas in terms of fatigue, there will be other difficulties if conventional gravity casting is used.
Indeed, in reference to FIG. 2, where a schematic cross-section of the mold is shown, it is necessary to provide a metal pin system to ensure that the stripping can occur in two directions D and D′ shown in FIG. 2, or a liner-holder pin system, which the necessary integration with a risering system would be extremely difficult to carry out.
For this reason, such an approach is almost never used.